The present invention relates to circuits and methods for frequency sensing. More specifically, the present invention relates to circuitry and methods for producing an electrical output which is a function of the frequency of a signal that has zero crossings. Without limiting the scope of applications contemplated for the invention, its background is described in connection with the field of electrical generator voltage regulation.
A generator of electrical power such as an alternator or rotary exciter typically has a field winding the direct current (DC) energization of which sets up a magnetic field in the generator and thereby controls the voltage of its alternating current (AC) output. A prime mover turns a rotor of the generator, and mechanical energy is converted into electrical energy for electrical power lines to a load in a conventional manner. The rotor speed (rpm) of the generator determines the AC frequency of its output. The AC output voltage is regulated by a voltage regulator such as a shunt static exciter (SSE) which senses the generator AC voltage and varies the DC current in the generator field winding to maintain the generator voltage level.
Motors and transformers that constitute the generator load have impedances that decrease with frequency and the resultant increase in current may damage these load components if the generator voltage is not reduced at lower frequencies. Thus, it is desirable that the generator output voltage not be held constant when generator rpm and frequency are reduced significantly. Therefore, some voltage regulators have an underfrequency rolloff circuit that senses the generator frequency and causes the generator voltage to be reduced at the lower frequencies.
Because of the importance of frequency sensing to generator voltage regulation and relaying in the electrical power field, new and improved types of frequency sensing circuits and methods with potential economic and performance advantages are of considerable interest. Also, there is a need for improved frequency sensing circuits with output characteristics that are relatively insensitive to temperature variations. In applications involving two or more generators operated in parallel, an economical type of frequency sensing circuit is needed to make the voltage-to-frequency characteristics of all the regulators for the generators exactly the same. Generally, there has been a lack of standardization of circuits to provide well-defined volts-per-Hertz performance and there is a need for a circuit which effects such standardization. There is also a need for circuits in which the volts-per-Hertz performance may be selectively adjusted without significantly increasing the complexity of the circuitry.